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Introduction to Green Roofs

In Essential Green Roof Construction, Leslie Doyle provides step-by-step instructions to build a green roof. By blending common sense with beauty, a green roof is a system of layers that work together to support plant life, insulate homes, and make the world a greener place. Today, Leslie introduces green roofs in an excerpt shared from her book, Essential Green Roof Construction

Excerpt from the Book

Our roofs, which keep us safe and dry, are subject to a range of environmental stresses. They are exposed to substantial rains and damaging hail, destructive heat and UV radiation, strong winds, and heavy snow. Our changing seasons and associated temperatures can wreak havoc on the waterproof membrane material used on many roofs, causing it to expand, contract, dry, crack, and then leak or break down.

Investing in a quality roofing membrane gives homeowners peace of mind in keeping their house protected. But why not go further? Why not utilize the roof space as a means to contribute to the biodiversity in your neighborhood and offer a protective place for pollinators or birds to feed? Why not reduce the impact of this space by reducing the heat it would normally reflect or capturing the rain that would run off of it? Why not do all these things? Why not make your roof do more?

You can. You can build a green roof. Green roofs, eco-roofs, vegetative roofs, vegetated roof assemblies, or living roofs— regardless of the name, they are all roof systems designed to support plant life. Green roofs provide a range of benefits for your site and your community, and they are an opportunity to turn a static surface into a buzzing tapestry of color.

A green roof under construction

Green Roof Systems

While some green roofs can be complex, others can be quite simple in design. Regardless of your design, for a green roof to succeed it must include the following five main elements:

  • a strong roof structure
  • a waterproof membrane
  • suitable growing media
  • good drainage
  • appropriate plants

Understanding how these elements come together will allow you to design and build the green roof you need. Throughout this book, I will provide you with visual examples to guide you.

You can build a green roof layer by layer (a built-in-place [BIP] system), or you can install manufactured products, such as trays that come pre-fitted with all the needed layers, often including pre-grown vegetation. You can also create hybrid systems. For example, layers can be installed on the roof with a pre-grown vegetation mat unrolled overtop, or you can install modules pre-filled with media and complete the planting on the roof.

Green Roof Categories

Green roofs fall into three categories: Extensive, Intensive, or Semi-Intensive. Extensive systems have shallow planting, making them inhospitable to many species. This means they support less plant diversity; however, they require less maintenance and upfront cost. These are the types of roofs most people think of when they think “green roof.” Intensive systems have few restrictions and can be designed as accessible public parks with soil depths deep enough to support trees. The middle ground of a semi-intensive roof typically allows for greater plant diversity than an extensive roof and therefore requires a little more maintenance, but they are not as resource heavy as intensive roofs. See Photos 1–3 in the Color Section for images of each kind. This book is focused on extensive roofs and semi-intensive roofs, but it also introduces food production roofs. Food roofs, rooftop farms, and edible roofs are all terms referring to green roofs that are built or modified for the production of food. These are often considered semi-intensive or intensive roofs due to the depth of the growing media and the frequency of maintenance that is required.

Extensive, semi-intensive, and small food roofs can easily be built and managed by an enthusiastic and competent DIYer on simple residential or outbuilding rooftops. Intensive roofs should be left to professional contractors, as they require an integrated approach with input from architects, engineers, landscape architects, and material suppliers.

Black and White photo of the flooring of a green roof being built

New Builds versus Retrofits

Green roofs can be built on both new and existing roofs. On new roofs, you have the freedom to build the roof structure based on your green roof design intent, whereas on an existing roof, your green roof may not exceed the existing structural capacity. If your existing roof does not allow for the added weight of a green roof, you can retrofit your roof to accommodate your loading needs; however, this can be a costly venture.

This book will give you everything you need to know about building a green roof but it does not directly show you how to design or build the roof structure underneath it. The typical green roof assembly does not include the roof structure itself. However, these two building components are highly interdependent. Reviews of local codes and/or a call to a structural engineer are necessary to ensure your structure is adequate for the amount of additional loading you intend to add.

How This Book Can Help You

The green roof industry is still relatively young and, therefore, still evolving. In North America, awareness and interest in green roofs has exponentially increased in the past decade and a half, and there is a growing body of research and publications. Whereas the early research was concerned with how the newly developed systems from Germany could be of use to our cities, research is now more focused on region-specific systems and designs for specific ecosystem services. Twenty years ago, the only books published on green roofs were intended to educate contractors on this new type of construction. Now, many books are published filled with award-winning designs of green roofs around the world. In addition, the industry is still learning many valuable lessons and developing products specifically for green roofs. As awareness and interest increase, many individuals are left wondering how they might build their own simple system.

Referencing lessons from these valuable resources as well as my experience in building green roofs for the past ten years, this book aims to provide the necessary information and planning steps for the Do-ItYourself builder. The lessons in this book are for homeowners building simple green roof projects; they are not intended to be applied to complex installations or used by commercial installers (who must strictly adhere to municipal building and safety codes and may have to follow manufacturer recommendations for warranty purposes). My goal for this book was to use common language that would not require a horticulture or construction background; however, I want you to be familiar with the key terms so you feel competent when ordering supplies or talking to professionals.

Sod being installed on a roof

In addition to terminology, you will find planning steps, installation and design considerations, plant and material choices, two installation examples, maintenance considerations, and an introduction to food production roofs. Consideration is given to the diversity of North American climates. Many details are provided so that regardless of the size of your build, you will have all the necessary planning preparation in front of you. However, if you are merely building a small green roof over a tool shed or a dog shed (in no way am I diminishing Spot’s need for well-being), you may not need to consider all the steps found in Chapters 3 and 4. They are still worth a read, however, so you can make informed decisions down the road.

Attention to Details

Racing through details can cause big problems, especially when you are dealing with water overtop of your home. Do not underestimate the ability of water to find small imperfections. In this book, you will find very detailed drawings illustrating the concepts; please review these carefully and apply the same level of detailed attention to any new ideas or variations you may wish to perform.

A finished green roof with plants beginning to grow

About the Author

Author Leslie Doyle

Leslie Doyle is the owner of Restoration Gardens Inc., a green roof design and installation firm. She is an instructor, a certified LiveRoof Installer, and is an accredited Green Roof Professional by the Industry Association, Green Roofs for Healthy Cities. Leslie resides in Toronto, Ontario.

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